In a single wavelength optical fiber transmission which was used prior to the establishment of a wavelength division multiplexing transmission system, a zero dispersion transmission had been generally considered to be preferable. Therefore, most of optical fiber transmission lines in use are designed so that a zero dispersion wavelength becomes a center of an amplifying a bandwidth of an optical amplifier.
Furthermore, in an optical transmission system designed for a wavelength division multiplexing transmission, each signal wavelength to be wavelength-division-multiplexed is usually set up so that it is contained within an amplifying bandwidth of an optical amplifier and, at the same time, it is also contained within a transmission bandwidth of an optical fiber. It is generally designed that an amplifying bandwidth of an optical amplifier coincides with a transmission bandwidth of an optical fiber.
When to use an existing optical fiber transmission line designed for a single wavelength transmission, the following problems would arise due to the smallness of a wavelength dispersion value of a signal wavelength. That is, the effect of Four Wave Mixing occurs extremely prominent and, thus, interference between channels greatly increases. It is difficult to increase the number of multiplexed wavelengths since enlarging the wavelength dispersion value is impossible. Furthermore, as the wavelength dispersion value is small, the difference of the wavelength dispersion value caused by a dispersion slope considerably affects transmission characteristics of each signal.
Even if an optical fiber transmission line suitable for a wavelength division multiplexing transmission is prepared without using the existing optical fiber transmission line, there still would be some problems as follows. It is preferable to make a channel interval of each signal as narrow as possible since a bandwidth of an optical amplifier is limited. However, on an optical fiber transmission line, there is the interaction between channels such as XPM (Cross Phase Modulation) and FMW, and, therefore, it is preferable to have a channel interval as wide as possible. Conventionally, a channel interval has been determined by an optical fiber transmission line and the number of multiplexed wavelengths by a bandwidth of an optical amplifier. To increase the number of channels in a wavelength division multiplexing system, up to the present, means for extending an amplifying bandwidth of an optical amplifier has been employed.